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Botany 350 - Midterm 2 (1998)


I. This past summer (1998) a cooperative farm raising organic dry beans found that 40 out of 100 plants had symptoms of white mold, caused by the ascomycete, Sclerotinia sclerotiorum. Before beans were planted into this field in May, they had measured 5 sclerotia/gram of soil. Knowing there is no secondary inoculum with this disease, they decided to clean up the bean debris (refuse) at the end of the season (September) to help lower disease in 1999.

a. Did the cooperative measure disease severity or disease incidence? (1 pt)

b. Differentiate between disease severity and disease incidence. (4 pts)





c. Is white mold a monocyclic or polycyclic disease? (1 pt)

d. Use the correct equation below to calculate the rate of disease progress (r or R) SHOW YOUR WORK. (3 pts)

Equation a: Equation b:
ln ( x ) = ln ( xo ) + rt ln ( 1 ) = QRT
1–x 1–xo 1–x



e. To what level would the cooperative need to reduce the number of sclerotia in order to only have 10% disease at 80 days after emergence the next year? SHOW YOUR WORK. (3 pts)





f. If the cooperative is successful in reducing the number of sclerotia to this level, but sustains 30% disease, how would you explain this? (3 pts)





g. Draw disease progress curves for the field in 1998 and in 1999. Label the axes and curves. (3 pts)







II. Draw a generalized disease cycle using all of the following terms. Some terms may be used twice. (8 pts)

Dormant period, colonization, penetration, symptoms, colonization, dissemination, secondary inoculum, primary inoculum
















a. Now add each of the following terms to your drawing in the step in the disease cycle to which it applies. Use each term ONLY ONCE. (6 pts)

Urediospore, teliospore, basidiospore, wind, pustule, haustorium

b. To what class of fungi does this pathogen belong? (2 pts)


c. Name a disease caused by this fungus. (2 pts)


d. Is this pathogen a biotroph or a necrotroph? (1 pt) Explain your selection. (2 pts)



e. Differentiate between a biotroph and a necrotroph. (4 pts)




III. Draw a disease triangle and label each corner (one word is sufficient). (3 pts)





a. Draw a polycyclic disease progress model. Label the axes of the graph. Now label the part of the curve limited by each factor (corner) of the disease triangle (4 pts).












b. Why does the curve flatten out at high levels of disease? (2 pts)




c. Pick one of the factors from the triangle, and give an example of a disease management tactic that could be used to further limit disease. (2 pts)






d. Draw a dashed line on your graph in (a) to indicate the result of your management tactic you selected in (c) (3 pts)


e. Name three methods of disease comparison used to describe epidemics. (3 pts)







f. Of the three which one would clearly illustrate the difference between the two epidemics you drew in (a) (2 pts).



IV. Matching. Place the letter from the left-hand column in the correct blank in the right hand column. One answer per blank (6 pts)

A. Polycyclic pathogens
B. Monocyclic pathogens

__________ One generation per season

__________ High birth rate pathogens

__________ Secondary inoculum

__________ Inverted j-shaped growth curve

__________ Exponential increase in disease

__________ Simple interest diseases


V. Confirmation of a diagnosis of a disease caused by a bacterium is frequently done with the serological test known as ELISA using either monoclonal or polyclonal antibodies.

a. What is the difference between an antigen and an antibody? (3 pts)



b. Differentiate between monoclonal and polyclonal antisera. (3 pts)




c. Caution needs to be taken in the interpretation of results from a serological tests. What is a false positive and how does it happen? (4 pts)




d. What is a false negative? Give two biological explanations for why false negatives happen (4 pts)




e. Of the two, false negative and false positive, which is of greater concern and WHY? (4 pts)




VI. Some pathogens have a very wide host range; e.g. they can infect a wide range of plant species whereas others have a narrow host range. One pathogen with a wide host range is Verticillium dahliae, cause of Verticillium wilt. In contrast, the pathogen that causes apple scab (Venturia inaequalis) causes disease only on apple. Explain why this is so. (6 pts)








a. What is a transgene? (2 pts)




b. What is the purpose of inserting the antibiotic resistance gene along with the gene of interest? (2 pts)




c. Give two advantages of engineering transgenic plants for resistance over conventional breeding (4 pts)